The use of insulation, weather stripping and other energy conserving products in the construction and renovation of buildings have successfully reduced the energy required for heating and cooling such buildings because the use of such products impedes the transmission of heat through walls, floors, and roofs. However, even with the construction and use of highly insulated buildings much energy is still unnecessarily wasted in heating and cooling due to heat transmission through windows.
The earliest response to this problem was the development of double glazed windows (R2--a thermal resistance of 2 hr-ft2-deg F/BTU) to replace single glazed (R1) windows. Such thermal window assemblies are usually constructed by placing sealed insulated glazing ("IG") units having the desired number of glazing layers in conventional window frames. "IG" is used in the art to refer to insulated glass units; however, because the applicants foresee glazing as being comprised of materials other than glass, the term "IG" units when used herein will not refer to a specific material but to insulated glazing units generally. While the addition of further glazing layers to the IG unit provides a moderate gain in insulating performance, it also adds weight and bulk to the window and reduces the transmission of light.
Since radiative transfer is a significant portion of heat transfer in a typical multi-glazed window, low emittance coatings have been developed which reflect long wavelength infrared energy and reduce window heat transfer. The addition of a low-emittance (low-E) coating to a double glazed IG unit provides the thermal efficiency of triple glazing (R3) without the additional weight, bulk and complexity.
Further improvements have been made possible by the addition of a low conductance gas to the space between the low-E glazings to reduce the other major component of heat transfer--conductive/convective heat transfer. For example, U.S. Pat. No. 4,459,789 discloses the use of Freon (Bromotrifluoromethane) gas in a sealed window assembly. In addition, some manufacturers offer argon gas filled IG units to provide about an R4 window assembly.
To provide further significant energy savings, sealed IG units need to be in the R5-R10 region. To achieve these performance values, such windows will have to further decrease both radiative as well as conductive/convective heat transfer.
One way of constructing such an R5-R10 IG unit would be to use 3, 4 or more glazing layers with low-E coatings and possibly argon gas. However, such units would be very bulky and heavy if glass is used as the inner glazing layers. This is due primarily to the optimum thermal spacing between glazing layers, although the thickness of the inner glazings will add to the bulk of the units, since the thermal gap is measured from an interior surface of one glazing to the facing surface of an adjacent glazing. For air or argon, for example, optimum gap widths between glazings are about 0.5 inches thick; smaller gaps are not thermally efficient. Thus, a quadruple glazed IG unit would be at least 2 inches thick and much heavier than a conventional double glazed window. While the weight could be reduced by using thin plastic films as the middle glazing layers, the necessary gaps would require a bulky, complex, and expensive mechanism and assembly process to stretch the thin plastic into a taut, wrinkle free layer. Further, the use of too many glazing layers may reduce light transmission to unacceptable levels. Finally, the necessary gaps between the glazing layers would essentially rule out the use of such windows in much of the large residential market which is oriented around sealed IG units ranging from 0.75 to 1 inches thick.
Several options for R5-R10 windows using new technologies are under development but are not yet commercially available. In one such R5-R10 window the space between the glazing layers in a low-E coated double glazed window is evacuated in order to reduce heat transfer. In another such R5-R10 window, the space between two glazing layers is filled with aerogel, a transparent insulating material. These new technologies, while providing R5-R10 insulation, also have inherent disadvantages due to the high costs of such windows, commercial production processes and final visual properties which are uncertain, and a large initial capital investment which would likely be required for start-up.
Thus, the need exists for R5-R10 thermal windows which can be easily constructed using commercially available low-E coatings, gases and glazing materials to provide lightweight, sealed thermal windows having dimensions and weight approximately the same as that of a conventional double glazed window. Such windows would greatly increase occupant comfort, reduce occurrences of condensation or frost on the glazing surface, and allow designers to meet more stringent building energy code requirements while providing design flexibility with respect to window area.